java - 如果多次打开 sqlite 则不会检索到数据

Question

当我向sqlite添加数据并第一次打开它时，会显示里面的数据。但是，如果我尝试再次打开它，即使里面有数据，也不会显示任何内容。我可以知道是什么问题吗？

执行 Activity 的类:

public class Watchlist extends ListActivity {

private ArrayList<String> results = new ArrayList<String>();
private String tableName = DatabaseHandler.TABLE_ITEM;
private SQLiteDatabase newDB;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    openAndQueryDatabase();
    displayResultList(); 
}

private void displayResultList() {
    TextView tView = new TextView(this);
    tView.setText("This data is retrieved from sqlite");
    getListView().addHeaderView(tView);

    setListAdapter(new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, results));
    getListView().setTextFilterEnabled(true);

}

private void openAndQueryDatabase() {
    try {
        DatabaseHandler db = new DatabaseHandler(this.getApplicationContext());
        newDB = db.getWritableDatabase();
        Cursor c = newDB.rawQuery("SELECT * FROM " + tableName, null);

        if (c != null ) {
            if  (c.moveToFirst()) {
                do {
                    String pid = c.getString(c.getColumnIndex("id"));
                    String name = c.getString(c.getColumnIndex("name"));
                    String price = c.getString(c.getColumnIndex("price"));
                    String date = c.getString(c.getColumnIndex("created_at"));
                    Log.d("pid",pid);
                    results.add("Name: " + name + "\n Price: " + price + "\n Date posted: " + date);
                }while (c.moveToNext());
            } 
        }           
    } catch (SQLiteException se ) {
        Log.e(getClass().getSimpleName(), "Could not create or Open the database");
    } finally {
        if (newDB != null) 
            newDB.execSQL("DELETE FROM " + tableName);
            newDB.close();
    }

}
}

数据库处理程序:

public class DatabaseHandler extends SQLiteOpenHelper {

// All Static variables
// Database Version
private static final int DATABASE_VERSION = 2;

// Database Name
private static final String DATABASE_NAME = "itemManager";

// table name
public static final String TABLE_ITEM = "item";

// Item Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_NAME = "name";
private static final String KEY_PRICE = "price";
private static final String KEY_CREATED_AT = "created_at";

public DatabaseHandler(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

// Creating Tables
@Override
public void onCreate(SQLiteDatabase db) {
    String CREATE_ITEM_TABLE = "CREATE TABLE " + TABLE_ITEM + "("
            + KEY_ID + " INTEGER PRIMARY KEY autoincrement,"
            + KEY_NAME + " TEXT, "
            + KEY_PRICE + " TEXT,"
            + KEY_CREATED_AT + " TEXT" + ")";
    db.execSQL(CREATE_ITEM_TABLE);
}

// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // Drop older table if existed
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_ITEM);

    // Create tables again
    onCreate(db);
}

/**
 * Storing item details in database
 * */
public void addItem(Items item) {
    SQLiteDatabase db = this.getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, item.getName()); // item name
    values.put(KEY_PRICE, item.getPrice()); // item price
    values.put(KEY_CREATED_AT, item.getDate()); // Created At

    // Inserting Row
    db.insert(TABLE_ITEM, null, values);
    db.close(); // Closing database connection
}

/**
 * Getting item data from database
 * */
public List<Items> getAllItems(){
    List<Items> itemList = new ArrayList<Items>();
    String selectQuery = "SELECT  * FROM " + TABLE_ITEM;

    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);
    // Move to first row
    if (cursor.moveToFirst()) {
        do {
            Items item = new Items();
            item.setID(Integer.parseInt(cursor.getString(0)));
            item.setName(cursor.getString(1));
            item.setPrice(cursor.getString(2));
            item.setDate(cursor.getString(2));
            // Adding item to list
            itemList.add(item);
        } while (cursor.moveToNext());
    }

    // return item list
    return itemList;
}

/**
 * return true if rows are there in table
 * */
public int getRowCount() {
    String countQuery = "SELECT  * FROM " + TABLE_ITEM;
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery(countQuery, null);
    int rowCount = cursor.getCount();
    db.close();
    cursor.close();

    // return row count
    return rowCount;
}

 // Updating item
public int updateItem(Items item) {
    SQLiteDatabase db = this.getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_NAME, item.getName());
    values.put(KEY_PRICE, item.getPrice());
    values.put(KEY_CREATED_AT, item.getDate());

    // updating row
    return db.update(TABLE_ITEM, values, KEY_ID + " = ?",
            new String[] { String.valueOf(item.getID()) });
}

// Deleting item
public void deleteItem(Items item) {
    SQLiteDatabase db = this.getWritableDatabase();
    db.delete(TABLE_ITEM, KEY_ID + " = ?",
            new String[] { String.valueOf(item.getID()) });
    db.close();
}

}

Answer 1

那么，关闭数据库之前不删除所有记录吗？

} finally {
    if (newDB != null) 
        newDB.execSQL("DELETE FROM " + tableName);
        newDB.close();
}

这可以解释这种行为。